The invention relates to an improved device for introducing a gaseous flow stream in an apparatus for granulating and/or coating particles in a spouted bed.
Spouted beds are beds of particles obtained in an enclosure having a vertical axis and fitted at the lower part with a frustoconical part terminating at a bottom opening through which there is introduced an upwardly directed gaseous flowstream. The upwardly directed gaseous flowstream maintains the particles in suspension and carries them along closed paths. Thus, the particles are directed upwardly by the gaseous flowstream in the neighborhood of the axis of the enclosure in the form of a spout or geyser. They move radially toward the wall of the enclosure and then slowly downward until they return to the tip of the frustoconical part. The gaseous flowstream creates, at the lower part of the bed in the frustoconical part, a pocket or pouch like area devoid of particles. Spouted beds of the type to which the invention is directed are described in French Pat. No. 1,125,461 filed on Mar. 11, 1955 and assigned to the National Research Council.
It is presently well known to use a spouted bed for granulating fluid products and hardening either by evaporating water or another solvent or by cooling, and/or for coating previously prepared particles or cores with these hardenable fluid products. According to the known granulating processes, a spouted bed of forming granules or of particles to be coated is maintained by means of a hot or cold upward gaseous flow stream with the hardenable product atomized in the gaseous flowstream beneath the spouted bed. The atomized hardenable fluid product is partly evaporated or cooled in the gaseous flowstream. More particularly, the evaporation or cooling occurs when it crosses the pocket or pouch like area which is devoid of granules, and subsequently deposits on the particles of the spouted bed in the lower part of the frustoconical part. The layer of atomized product continues evaporating or cooling until solidification is obtained as the particles are carried upwardly again by the gaseous flowstream in the axial spout, and while they return downward along the walls of the enclosure. When the particles again reach the lower part of the frustoconical part of the enclosure, they receive a new coating of atomized product. Thus, granulation and/or coating is accomplished by successive deposits or layers of atomized hardenable product. Such a granulating process is described in U.S. Pat. No. 3,231,413, filed Jan. 25, 1966 and assigned to Potasse et Engrais Chimiques.
The apparatuses used up to now for granulating in spouted beds include an enclosure having a vertical axis and having a lower frustoconical part with a cylindrical part on top. For continuous operation, the enclosure has at the upper part of the spouted bed, a laterally extending passage for continuously drawing the granules off. The described upwardly directed gaseous flowstream is introduced through the small base of the frustoconical part of the enclosure. The spray nozzle for the substance to be granulated, or for the coating substance, is positioned in the vicinity of the small base of the frustoconical part of the enclosure.
In the apparatuses proposed up to now, the junction between the duct pipe for introducing the gaseous flowstream, and the base having a diameter d, of the frustoconical part, make up a converging-diverging system. The duct pipe for introducing the gaseous flowstream has a diameter larger than d and also terminates in a frustoconical part, the small base of which has a diameter equal to d. It was thought that by using a converging-diverging system in which the gaseous stream is noticeably accelerated, the spraying of the fluid hardenable product and therefore, the granulating and/or coating would be facilitated. However, it was found that such a device did not perform satisfactorily when operating continuously. More particularly, in apparatuses having a large diameter, the gaseous flowstream is not homogeneously distributed as a result of the dynamic pressure being greater in the vicinity of the walls of the frustoconical part than in the vicinity of the axis. Therefore, large agglomerated particles are formed in the lower part of the frustoconical part and often block up the spouted bed after 2 or 3 hours of operation.
Another type of prior art device and process is disclosed in U.S. Pat. No. 3,110,626 which is assigned to Minnesota Mining and Manufacturing Co. which uses a venturitype structure to control coating and granulation which includes a guidance element supported within either a conical or tubular structure. The guidance element can have one of a variety of shapes and cooperates with a multiply diverging and converging structure to improve particle coating and granulation. This type of device has the disadvantage however, that it is very expensive and more complicated to manufacture than the device and process of the present invention.